Automotive dynamometers,  the software!

You do not NEED any computer software to calculate simple BHP or KILOWATTS from an engine.

For example, a simple BRAKED dynamometer can be made with nothing more than a spring balance!

I used to measure the Horsepower (or Kilowatts) that my model aircraft motors produced simply by mounting the engine on a mount that "swivelled".  It was arranged so that the axis the engines mounting was fixed to turned freely in line with the crankshaft.  

Now when the engines were run up, they turned the propeller one direction, which formed the BRAKE, (an air brake in this case)  and they tried to "twist" themselves the other way.   This twist is the torque they were making.  A "lever" attached to the mount that was exactly 12 inches long, and a spring balance from a fishing shop stopped its rotation.  The fishing balance read in pounds weight (lbs) and measured this twisting action.

This "TWIST" is actually the TORQUE that the motor produces.   It was turning it's propeller at 10,000 RPM.   Its easy to check with a cheap photo sensor tachometer aimed at the propeller - available from any model airplane shop!

This is ALL THAT'S REQUIRED to measure the engines power because TORQUE x RPM = Horsepower! 

That "lever" attached to the swivelling engine above, was a FOOT long.  If the spring balance read lets say, 2lb when its running, then its making 2 lb-ft of torque!  It was doing 10,000 RPM so now we can calculate the exact Horsepower it was making.   We have all the information required.  But only for one single RPM point.  We could only plot one point on our graph!  Every other point would need a bigger or smaller braking force -  or "propeller" in this case!

So for big car or bike dynamometers we either have more complex "brakes" (either water, electric. disk brake etc) that is variable.  In this case the SOFTWARE samples the TORQUE measured at the brake by using a "LOAD CELL"  which replaces our fishing balance,  which gives a continuous variable TORQUE reading. (usually as a small DC voltage) and the computer logs this and it logs the engines RPM's at the same time. 

Now the Dynamometer Software can display a graph of the engine speed & Torque.   Because it can do this, it can also calculate horsepower using a simple formula, because torque x RPM = horsepower!  So it can also display RPM & Horsepower on the graph.  Now this is simple enough.

It also needs to have a method of storing and retrieving all the runs, and customers, and other information.  It also needs to know the air intake temperature, the atmospheric (barometric) pressure, the relative humidity so it can calculate the corrected power curves.   It also needs to be able to control the load (how much "brake" during the run. It has to do this to allow x number of RPM rise per second during the run.  And much more!  The software is the complex part of a dyno, and without it the Dynamometer is an extremely basic device.

INERTIA DYNAMOMETER SOFTWARE

This works a little differently. We have no load cell to give us a torque reading.  Here we simply accelerate a heavy steel drum, of known rotational inertia.  This is usually calculated from the drums dimensions on well designed systems, so this is an extremely precise value!   A high accuracy dual hall effect "gear position" sensor is required (or similar) that reads each 360 degree rotation of this drum.

On GOOD SYSTEMS this drum is timed by a separate extremely accurate time base - independent from the computer board.  It also MUST read real time engine RPM during each revolution. The external circuit then sends this data "packet" to the PC.  See note at end

It sends this data to a PC which can then calculate the power / torque / speed / gear ratio / etc and store and display each run / customer for future reference.

>>> Side note!  CAUTION!!!   A normal PC or a PC based card like a sound card at 44k simply CANNOT be used for this drum timing because it simply is not accurate enough, and the PC time base "wobbles" slightly. Meaning that MUCH data averaging has to be done to display a "smooth" graph.  But then all the REAL small changes cannot be seen!  In some extreme cases of dodgy dynos I have examined even gear changes were hard to spot!  And people SELL these as "Dynamometers"!!! Frankly I was amazed.  Be warned, MOST cheapo inertia Dyno systems are like this!   Don't buy one! Things to spot...  Graphs that get zig zaggy the more the speed increases, usually at around peak power...  Graphs that are less smooth in top gear than in say 2nd gear Roll on tests.  Graphs that "lean" backwards towards lower RPM at the end of the run.  This should IS IMPOSSIBLE! Its actually caused by lots of data averaging, needed because of very poor drum timing.  LOTS of so called dynos do this...  I will not name names but take care what you use, or BUY!    

Oh and for lots of reasons NEVER buy a dynamometer that "appears" not to need an "actual" RPM hook-up!   All it really means is that the manufacturer could not work out how to read real time RPM as well as to time the drum extremely accurately! Its a hard trick!   But you NEED real time RPM data as tyres slip, tyres expand at speed increasing the gearing, clutches slip, and some bikes and cars  are automatic!  Any form of "fixed gearing" ratio, or gearing database, or "grabbed" rpm that fixes a set ratio is absolutely useless!  Again BE VERY VERY CAREFUL of what you buy here!  I have only seen myself two systems done properly, and that is my own DynoPower Dynamometer Systems, and the US DynoJet Dynamometer systems.

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